How To get 100% Bettery health again?

That is not possible.

Batteries degrade over time with usage due to chemical aging, that's just how they work.

You can expect about a 1% drop per month with average use on an iPhone. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

iPhone Battery Replacement.

iPhone Battery Repair & Replacement - Apple Support

About Lithium Ion Batteries

iPhone battery and performance - Apple Support (IN)

Here are some tips on maximizing battery life.

Batteries - Maximizing Performance - Apple

You can't without getting it replaced; Apple's only obligated to do this when it goes below 80%. They wear out as they age.

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Once your battery health drops below 100%, the only way to achieve 100% health again is to replace the battery.

Of course, 99%, 98% … is perfectly acceptable.